1. Field of the Invention
The present invention relates to a method of manufacturing a plasmon generator for use in thermally-assisted magnetic recording where a recording medium is irradiated with near-field light to lower the coercivity of the recording medium for data writing.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, the thin-filth magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a recording medium having high coercivity. When writing data, a write magnetic field and heat are simultaneously applied to the area of the recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating the near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near a medium facing surface of the slider.
U.S. Pat. No. 7,911,883 discloses a technology in which the surface of the core of a waveguide and the surface of a plasmon generator are arranged to face each other with a gap therebetween, and evanescent light that occurs at the surface of the core based on the light propagating through the core is used to excite surface plasmons on the plasmon generator. Based on the excited surface plasmons, near-field light is produced.
The plasmon generator has a front end face located in the medium facing surface. The front end face includes a near-field light generating part which generates near-field light. The surface plasmons excited on the plasmon generator propagate along the surface of the plasmon generator to reach the near-field light generating part. As a result, the surface plasmons concentrate at the near-field light generating part, and the near-field light generating part generates near-field light based on the surface plasmons.
In order to reduce the track width of a recording medium for higher recording density, it is required to reduce the near-field light in spot diameter at the recording medium. To achieve this, it is required to reduce the width and height of the front end face of the plasmon generator. Note that the width of the front end face refers to the dimension of the front end face in the track width direction, and the height of the front end face refers to the dimension of the front end face in the direction in which the tracks extend. The width and height of the front end face are both preferably 40 nm or less.
The plasmon generator is typically manufactured by photolithography. The following first and second methods are conceivable as examples of methods for manufacturing the plasmon generator by employing photolithography. In the first method, a metal film is etched into the plasmon generator by using an etching mask formed by photolithography. In the second method, a dielectric layer is etched to form therein a groove by using an etching mask formed by photolithography, and the plasmon generator is formed in the groove.
With the first method, it is difficult due to the limitations of photolithography to reduce the width of the front end face of the plasmon generator to 40 nm or smaller. Furthermore, with the first method, even if the width of the front end face of the plasmon generator could be reduced to 40 nm or smaller by etching, part of the plasmon generator would be susceptible to being stripped off from the underlying layer.
With the second method, it is also difficult due to the limitations of photolithography to reduce the width of the groove, and as a result, it is difficult to reduce the width of the front end face of the plasmon generator to 40 nm or smaller.
U.S. Pat. No. 7,911,883 discloses a method of forming a plasmon generator that is V-shaped in cross section parallel to the medium facing surface. In the method, a groove that is V-shaped in cross section parallel to the medium facing surface is initially formed in a dielectric layer, and a dielectric film is then formed in the groove. Thereafter, the plasmon generator is formed in the groove. With this method, however, it is difficult to form with stability the plasmon generator so that its front end face has a width of 40 nm or smaller. The width of the front end face disclosed in U.S. Pat. No. 7,911,883 falls within the range of 50 to 350 nm.
The reason why it is difficult with the aforementioned method to form with stability a plasmon generator whose front end face is 40 nm or smaller in width is as follows. The front end face of the plasmon generator formed by the aforementioned method has a nib at a position closest to the bottom of the groove. Such a front end face of the plasmon generator greatly varies in shape of the portion near the nib due to variations in the accuracy of the process for forming the groove or the process for forming the dielectric film. If the aforementioned method is employed to form a plasmon generator whose front end face has a width of 40 nm or smaller, the main part of the front end face will be occupied by the portion that greatly varies in shape as mentioned above. Accordingly, with this method, it is difficult to form with stability a plasmon generator whose front end face has a width of 40 nm or smaller.